The problem of too many men and not enough women in Chinese villages is likely to become much worse, a leading researcher in the field has warned.

China has 32m more men aged under 20 than women, according to a paper published last month by Therese Hesketh, of University College London. Her latest research suggests that rural areas – where the imbalance is at its greatest – will be further affected because women are "marrying out" into cities.

"It looks as if high sex ratio areas, particularly in central China, are likely to get worse," she said. "Because of migration, we are hearing again and again that women are going to urban areas and staying. In rural areas that will exacerbate the sex ratio very markedly.

"In the past, migrants have tended to go back home to permanently settle. But women [now] are finding partners in urban areas and not going back. Men are unable to do that. Urban women will not marry a migrant man; men can't marry up."

Hesketh said her team had not set out to investigate the problem but rural men were frequently raising the issue. The researchers are now embarking on a fuller study to try to assess the extent of the problem.

"I think this is very new. If [official] planners know about it, they're not talking about it," she added.

Farmers have been migrating to cities since the 90s, but Hesketh believes the numbers have soared in recent years and it has become easier for migrants to settle in the cities.

Her paper in the BMJ last month, which was coauthored by Professor Wei Xing Zhu, of Zhejiang Normal University, and Professor Li Lu, of Zhejiang University, said that China has 119 male births for every 100 girls, compared with 107 to 100 in industrialised countries.

"We have never seen these sex ratios before. There have been populations where the sex ratio has altered, but never to this degree," Hesketh added.

The biggest male-female gap is among under-fours, meaning the problem of "bare branches" – young men who can't find partners or have children – is likely to increase.

The gap is greater in provinces that allow couples to have a second child if the first is a girl. Among second-born children, boys outnumbered girls by 143 to 100. The data is based on the 2005 census.

The paper suggested that sex-selective abortion – banned in China but widely practised – was the main reason for a higher number of males. While noting that many nations have disproportionate sex ratios, it added: "In the era of the one-child policy the fact that the problem of excess males in China seems to outstrip that of all other countries is perhaps no surprise."

Hesketh said many families wanted males for practical reasons; to work the land and support them in their old age, as well as because they were traditionally viewed as more valuable.

But she added: "We think the sex ratio at birth is probably peaking now. People are recognising that if they have male children who can't marry when they're older that it's not a good thing."

The government has expressed concerns that too many men could lead to social instability and is expanding programmes that encourage people to have female children.